Hurricane seasons warm up

Possible links between hurricane formation and global warming are a contentious issue in climate policy. And last week's Hurricane Katrina in the United States has fanned the flames.

The depth of the divide between supporters and sceptics became apparent in January, when US meteorologist Chris Landsea resigned from the Intergovernmental Panel on Climate Change (IPCC). He was protesting against statements made by his panel colleague, Kevin Trenberth, who had supported a link between warming and storms in a press conference.

Trenberth, head of the climate analysis section at the National Center for Atmospheric Research in Boulder, Colorado, tried to set the record straight in an article published in Science in June1.

Owing to large natural variability, trends in hurricane frequency are indeed difficult to attribute to climate change, but human influences on the environment probably affect hurricane intensity and rainfall, he argued (see ' Trouble brews over contested trend in hurricanes' ).

Solid analysis

“You shouldn't score cheap points by turning a scientific question into a political one.”

Roger PielkeUniversity of Colorado in Boulder

Trenberth's view is supported by the most recent and solid analysis of hurricane destructiveness over the past 30 years, by leading US hurricane researcher Kerry Emanuel of the Massachusetts Institute of Technology in Cambridge, Massachusetts.

In his Nature paper2, Emanuel concludes that "future warming may lead to an upward trend in tropical cyclone destructive potential, and ... a substantial increase in hurricane-related losses in the twenty-first century."

The main cause of all this may be increased sea surface temperatures; these are the most important variable affecting hurricane formation. Extra warmth means there is more energy available to a storm, and more water is likely to be sucked up into the clouds. North Atlantic surface temperatures have been significantly above average for at least a decade, a trend many scientists agree must be associated with global warming.

In early August, the Gulf of Mexico, was a striking 2-3 °C warmer than it usually is at this time of year. Katrina sucked out so much heat energy from the Gulf that water temperatures dropped dramatically, in some regions from 30 °C to 26 °C, after the storm had had passed. Sea temperatures are likely to remain high until October.

The IPCC came to the tentative conclusion that hurricanes might be on the rise in their most recent assessment report, published in 2001. It stated: "There is some evidence that regional frequencies of tropical cyclones may change.... There is also evidence that the peak intensity may increase by 5% to 10% and precipitation rates may increase by 20% to 30%."

But the panel adds that the certainty of these statements is low: lower than our understanding of air temperature changes, for example. "There is a need for much more work in this area to provide more robust results," it says.

Weighing in

Naturally, the bulk of the media are less reserved. "The hurricane that struck Louisiana yesterday was nicknamed Katrina by the National Weather Service. Its real name is global warming," opened one opinion piece in the Boston Globe on 30 August.

Such statements are not pure invention, but most scientists are made uncomfortable by this specific kind of attribution. Even in the presence of a statistically robust trend, it is unscientific to attribute a discrete atmospheric event to climate change.

"No matter what one's opinion is on global warming and hurricanes, you shouldn't score cheap points by turning a scientific question into a political one," says Roger Pielke, director of the Center for Science and Technology Policy Research at the University of Colorado in Boulder.

Attempts to use hurricanes to justify energy policies to mitigate climate change may prove counterproductive, he fears. "This would only provide a great opening for criticism of the underlying scientific reasoning."